1. Field of the Invention
This invention generally relates to heterodyne receivers, and is more particularly concerned with a calibratable heterodyne receiver which can be adjusted to a calibration mark or equivalent indicia of an indicating or recording device connected to the output of the receiver during feeding of a calibration alternating current and adjustment of amplifier gain. The calibration alternating current frequency which is adapted each time to the tuning frequency is obtained by modulating the output voltage of a heterodyne oscillator of the receiver with an auxiliary voltage at a frequency which corresponds to the intermediate frequency.
2. Description of the Prior Art
An arrangement which operates according to this general principle for calibrating a selective transmission level meter is known from the German DT-AS 1,240,989. Such heterodyne receivers, which are supplied with their own auxiliary voltage generator for the purpose of calibration, have the disadvantage that every deviation of the frequency of the auxiliary voltage from the desired value, which is larger than half of the bandwidth of the pass band of the intermediate frequency band pass filter of the receiver leads to a shifting of the converted frequency of the calibration alternating current into the step band of the intermediate frequency band pass filter, and accordingly causes faulty calibration. A frequency deviation of this magnitude can occur at any time, particularly with highly selective heterodyne receivers due to the occurrence of drift in the auxiliary voltage generator.